Manufacturers of internal combustion engines develop engine control strategies to satisfy customer demands and meet various regulations for emissions and fuel economy. One such engine control strategy comprises operating an engine at an air/fuel ratio that is lean of stoichiometry to improve fuel economy and reduce greenhouse gas emissions. Such operation is possible using compression-ignition (diesel) and lean-burn spark-ignition engines. When an engine operates with lean (excess oxygen) air/fuel ratio, the resultant combustion temperature is lower, leading to decreased engine-out NOx emissions; However, commercial application of lean-operating engines is limited due to lack of effective methods to remove NOx under a lean exhaust condition. Thus, efficient reduction of nitrogen oxides (NOx=NO+NO2) from diesel and lean-burn gasoline exhaust is important to meet future emission standards and improve vehicle fuel economy.
Reduction of NOx emissions from an exhaust feedstream containing excess oxygen is a challenge for vehicle manufacturers. By way of example, it is estimated that compliance with Bin 5 regulations in the United States may require an aftertreatment system capable of 70-90% NOx conversion efficiency on the FTP (Federal Test Procedure) cycle based on currently anticipated engine-out NOx levels. For practical application, the conversion efficiency must be obtained at a low temperature operating range (e.g., 200-350° C.) occurring during the aforementioned FTP cycle and at a higher temperature operating range (e.g., 450-550° C.) occurring during a high speed test cycle (e.g., US06 federal test procedure).
Several potential aftertreatment systems have been proposed for vehicle applications. One approach comprises using an aftertreatment system including injecting a NOx reductant, e.g., urea, upstream of an urea-SCR catalyst, to reduce NOx to N2. Use of urea as a reductant necessitates a urea distribution infrastructure and an on-vehicle monitoring system for this secondary fluid, and may have potential problems in cold weather climates due to the relatively high freezing point (−12° C.) of the urea solution. NOx storage catalysts typically require large catalyst volumes, large amounts of platinum-group metals and low sulfur fuel for efficient storage operation. Such systems require periodic catalyst regeneration involving fuel injection to generate high exhaust gas temperatures and injection of reductants to regenerate the storage material of the catalyst.
Selective catalytic reduction of NOx using hydrocarbons (HC—SCR) has been studied extensively as a potential alternative method for the removal of NOx under oxygen-rich conditions. Ion-exchanged base metal zeolite catalysts (e.g., Cu-ZSM5) have typically not been sufficiently active under typical vehicle operating conditions, and are susceptible to degradation by sulfur dioxide and water exposure. Catalysts employing platinum-group metals (e.g., Pt/Al2O3) operate effectively over a narrow temperature window and are highly selective towards N2O production.
Catalytic devices using alumina-supported silver (Ag/Al2O3) have received attention because of their ability to selectively reduce NOx under lean exhaust conditions with a wide variety of hydrocarbon species. The use of partially oxidized hydrocarbons (e.g., alcohols) over Ag/Al2O3 allows reduction of NOx at lower temperatures. However such reductants are unavailable on-board the vehicle. Previous HC—SCR over Ag/Al2O3 catalysts utilized light hydrocarbons (e.g., propene, propane) and heavier fuel-component hydrocarbons (e.g., octane, decane) as a reductant. NOx reduction using lighter hydrocarbons present in engine exhaust as the combustion products yield conversion at higher temperature, but for Ag/Al2O3 catalysts to be considered as candidates for practical use, the NO reduction must be shifted to a lower temperature region and the fuel on-board the vehicle must be used as the reductant.
Therefore, there is a need for an effective method and apparatus to selectively reduce NOx in an exhaust gas feedstream for vehicles and other applications of lean-burn internal combustion engines.